1. The Field of the Invention
The present invention generally relates to apparatus and methods for transferring electrical power to and from electrical energy storage devices, and more particularly to a method and circuit for charging and discharging a plurality of parallel connected batteries connected to a plurality of DC to DC converters.
2. Description of Related Art
Battery-powered electric vehicles (EVs) are in high demand in automotive markets around the world. Each year thousands of EVs are manufactured, and in like manner their batteries are mass-produced from lithium and other raw materials. Because these batteries are used in vehicles, they have high energy storage capacity, high temperature resistance, strict safety requirements, and reliable energy discharge profiles. As their host vehicles age, these valuable batteries may end up in landfills and junkyards. Lithium prices are still relatively low, and the cost of recycling a battery is significantly higher than the value of its components, so battery recycling programs that reclaim the raw materials for reuse are few and far between. Some government programs offer this kind of battery recycling, but they mainly relate to cellular phone or notebook computer batteries and, even more importantly to the industry, tend to operate at a loss. Governments can justify their recycling programs' inefficiency by touting projected environmental benefits, but commercial and industrial enterprises typically cannot afford such operations. To these enterprises, a preferable solution would be to repair and reuse the batteries they already own, without resorting to complete reconstructions, or to resell them for use in other applications.
Unfortunately, EV batteries are not interchangeable between EVs, and are difficult to adapt to other applications as well. Each EV has different battery requirements and battery management hardware and software, and it is difficult to safely connect a used battery to a load because the sizes, capacities, voltages, and other essential characteristics vary from one discarded used battery to the next. If these batteries could be connected notwithstanding their differences, banks of high-quality EV batteries could be recycled for various energy storage needs. Notebook computer batteries and batteries of other devices could be likewise recycled and repurposed for different applications.
High-capacity storage applications are a prime field to use these EV batteries. This would entail connecting the batteries in parallel, meaning the positive terminals are all connected to each other, and the negative terminals are similarly connected, because parallel topology allows each of the batteries to contribute power to the load simultaneously for a longer duration and at a lower voltage than if they were connected in series. There are limited practical circuit designs where used or dissimilar batteries are connected in parallel because of difficulties in charging and discharging the batteries effectively. Connecting dissimilar batteries in parallel results in improper polarization of the electrodes of one or both batteries, reduced battery life, possibility of rupture, reduced electrical storage capacity, and, disproportionate utilization of battery capacities.
Some systems in the field of battery circuit design have addressed these issues by connecting diodes to the batteries to prevent circulating or back-flowing currents when the batteries are connected in parallel and to avoid a situation where the discharge of one battery charges another. In this case, the forward voltage drop of the diode can be large compared to the battery voltage, and the use of the diodes introduces losses in the energy transfer from the batteries.
Some systems connect DC to DC converters to the batteries in order to alter the current to voltage ratio at each battery to match an average level, thereby minimizing back-flowing currents. DC to DC converters are a type of power converter used to bridge a power source of electronic equipment to a load and are especially used in connection with portable equipment powered by batteries. The converters are circuits most often configured to convert a source of direct current (DC) from one voltage level to another. Switched-mode power converters convert voltage by storing input energy temporarily and then releasing that energy at a different voltage. The storage may be in either magnetic field storage components (e.g., inductors) or electric field storage components (e.g., capacitors).
When connected in parallel, systems with DC to DC converters are interdependent among the parallel branches of the circuit. This means that the DC to DC converter of a given branch relies on measurements of signals from the converters of other branches in order to keep the voltage output of the given branch equivalent to the other branches. For energy storage devices like batteries, voltage is one of the measured quantities used to indicate state of charge, and is also a quantity that is forced to match the other branches such that every branch has the same voltage. Hence, new parallel elements must have a voltage-to-charge profile that is similar to the other paralleled elements or there will be differences in the state of charge of the various branch-connected storage devices resulting in inefficiency over charge and discharge cycles or damage to the parallel elements. Voltage or some other parallelable quantity such as rotational velocity must always play this role in any system of parallel-connected energy storage devices.
Other techniques for connecting multiple batteries in parallel include oversizing the battery cell capacity to account for loss, or providing a relay and switch matrix to compensate for multiple battery cell failures. Still other designs incorporate elements to reduce wear on the parallel-connected batteries by feeding shared load current readings in a feedback loop to battery voltage converters to correct irregularities in each battery's current output. These solutions are also interdependent among the parallel branches of the circuit, and new parallel elements must have an energy discharge profile (or some other measureable quantity-to-discharge profile) that is similar to the other paralleled elements or there will be unused capacity of the batteries and decreased efficiency.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments or only using measurables such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be beneficially utilized.